Adaptations of Marine Organisms to Environmental Challenges edit
Hypoxia edit
Hypoxia occurs when the dissolved oxygen concentration in a body of water is low, between 0-2 mg/L.[1] At 2 mg/L, the body of water has about 30% oxygen saturation.[1] When the dissolved oxygen concentration of a body of water reaches 0 mg/L anoxia occurs, as the body of water contains no longer contains any oxygen.[2] Possible causes of Hypoxia can include strong vertical stratification in the water column[3], or eutrophication, which can result from human agricultural practices.[1]
Hypoxia can act as a limiting factor for the growth, reproduction and distribution of aquatic species[3]. Other possible effects of hypoxia on marine life include mortality of marine organisms and loss of benthic ecosystems, making hypoxia a major threat to costal ecosystems.[3] Benthic organisms that live the farthest away from the region of gas exchange with atmospheric oxygen are especially vulnerable to hypoxia.[4]
Adaptations for Surviving Hypoxia edit
Marine organisms possess several adaptive strategies that can help overcome the risks of hypoxia [3]. These strategies may use on a combination of physiological and biochemical responses to enhance the oxygen uptake from the environment or to prevent stressful behaviours from occurring[3]
As oxygen saturation decreases, a common physiological response is to breathe in more air to get as much oxygen as possible.[3] Marine fish achieve an equivalence to this by increasing water flow through their gills to help maintain oxygen levels.[5] This mechanism occurs through opercular movement and the organisms to increase their ventilatory frequency (VF)[3]
However, these processes are very energy demanding. While organisms may survive hypoxia using these adaptations, they may exhibit a decrease in fitness and activity due to a lack of available energy [3][5].
Two organisms that have adapted to hypoxic conditions are the Peruvian rock sea bass (Paralabrax humeralis) and the Peruvian grunt (Anisotremus scapularis)[3]. These organisms live in Peruvian Coastal waters, where a shallow oxygen minimum zone and strong vertical stratification result in low concentrations of oxygen[3]. The Peruvian rock sea bass will experience a reduction in activity and metabolism during hypoxic conditions, which will allow it to tolerate hypoxic events for long periods of time.[3] The Peruvian grunt is less tolerant to hypoxic conditions, so it has adaptations that aim to avoid hypoxic areas instead of enduring them.[3]
Hydrological Disturbance edit
A hydrologic disturbance is defined as an abrupt shift that changes the previously established hydrologic function of a system.[6] One common type of hydrological disturbance is drying, where water disappears across an increasingly large surface area.[7] This can result in a loss, isolation and fragmentation of aquatic habitats.[7] The disturbance to aquatic communities through loss and fragmentation of habitat makes drying a prominent threat to aquatic invertebrate biodiversity.[8]
Drying presents a major ecological challenge for benthic organisms.[7] It makes their habitat completely inhabitable or reduces the habitable area into a few aquatic patches.[7] These patches serve as a place refuge for the organisms where the risks of disturbance are lower than those in the surrounding area.[9] While these patches are helpful to organisms, they are small in size and are not abundant in occurrence.[7]
One region that experiences aquatic habitat drying is the Maranchery Kole wetland in India.[10] The wetlands in this region often undergo dewatering, which is a transformation a body of water with aquatic organisms into a terrestrial landscape for agricultural activities.[7] This creates a drying effect in that area and leaves only small patches of water as a habitat for aquatic organisms.[7] Climate change and dewatering practices are causing a disappearance of water in many aquatic ecosystems, and in regions such as India prevalence and severity of hydrological disturbances is expected to increase in the future.[11]
Adaptations to Hydrological Disturbance edit
Organisms may use many different traits to combat the risks associated with hydrological disturbance.[7] Worms in the Nais, Pristina, and Pristinella genera reproduce asexually and construct coccoons that are resistant to desiccation to help retain moisture and prevent drying.[7] Other organisms, such as the dragonflies and damselflies in the Crocothemis, Ischnura, and Libellula genera, will actively disperse and fly between patches of water to lay their eggs.[7] Midges in the genera Chironomini and Tanytarsini undergo asexual reproduction and passive aerial dispersal to survive the harsh effects of drying.[7]
Absence of Light edit
Down-dwelling light is the most common type of light found in large bodies of water.[12] This light originates from the sun and streams down vertically through the water.[12] Down-dwelling light decreases as water depth increases, and past 800 meters there is not enough light to be perceived by the organisms that live there.[12] This lack of light means that photosynthetic organisms cannot live at this depth, making food difficult to find.[12]
Adaptations to an Absence of Light edit
Bioluminescence is used by some marine organisms to overcome the challenges presented by a lack of light.[12] Bioluminescence occurs when an organism produces light in it's body.[12] This allows organisms to attract prey, to camouflage themselves, and to communicate with others.[12] Light produced by bioluminescent species is often blue-green in colour.[12] This happens because light with shorter wavelengths, such as red light, is often absorbed at that depth and cannot be seen by organisms.[12]
One species that relies on bioluminescence to survive is the lanternfish (Myctophidae family).[12] There are two main bioluminescent organs used by lanternfishes. These are the ventral photophores and the ventro-lateral photophores.[12] The arrangement of these photophores may vary between species.[12]
The reaction used to produce light in Lanternfish is the luciferin-luciferase reaction .[12] This reactions occurs inside of photocytes, where the molecule luciferin is oxidized by the enzyme luciferase, and light is produced as a result.[13] The blue-green colour seen in lanternfish light is a result of the parabola-shaped reflector on the inside of the photocytes.[13]
The light produced by lanternfish species is often weak and intermittent.[12] Lanternfish can produce many different patterns of light, however the the light produced by this species is often weak and intermittent.[12]
Organisms that rely on bioluminescence to survive deep waters must have eyes that are well attuned to detecting these light patterns.[12] Adaptations that are common among organisms that use bioluminescence include big eyes, reflectors behind the retina, a large number of rod cells, and gaps between the lens and the pupil, which allows light to directly enter the retina.[12] These adaptations allow the organisms to detect light and overcome other factors that effect vision in deep waters, such as light intensity, depth, and the turbidity and the size of dissolved particles in the water.[12] Many deep sea fish species have also formed retinal pits, which are areas of the retina with a high concentration of receptors. This increases image resolution, and improves the organism’s ability to detect movement.[12] The eyes of some species of Dragonfish are able to detect far-red light, allowing them to detect prey or communicate without being glimpsed by predators.[14]
There are other adaptations that organisms use to survive the absence of light in deep waters.[12] Deep sea organisms may rely on senses other that vision to navigate their surroundings, communicate, and find prey.[12] These senses may include olfaction, taste, electroreception, and detecting changes in motion and pressure through the use of lateral lines.[12] Some cnidarians have been shown to exhibit biofluorescence, where an organism will absorb and re-admit light instead of producing it themselves. Like Bioluminescence, this can also be used to attract prey.[15]
High Pressure edit
As the depth of a body of water increases, the pressure of the water increases, which can be dangerous for many marine organisms.[16] Hydrostatic pressure in the deep sea may increase by 1 atmospheric unit for every 10 meters of depth, and deep sea organisms in the Mariana’s Trench may have to withstand pressure of up to about 110 mega pascals. [16] High pressure can harm organisms at a molecular level as an increase in pressure can cause biomolecular processes to undergo changes in bond length and hydration levels.[17] The increase in pressure can disfigure and damage proteins, stiffen cellular membranes, and block the use of substrates and enzymes sites.[16]
Adaptations to High Pressure edit
Organisms use many methods to prevent proteins from being damaged due to pressure changes.[16] One is an intrinsic adaptation, in which proteins shift their amino acid residues to increase their stability and flexibility.[16] Organisms may also use molecular or chemical chaperones, molecules that are responsible for stabilizing the proteins.[16] These include osmolytes and piezolytes.[16]
Osmolytes such as glycine can allow deep sea fish to fight pressure and osmotic gradients in the high salinity deep sea.[16] This occurs as glycine allows the organism to stay isosmotic with the surrounding salt water.[16] Osmolytes may also protect the proteins against other conditions, such as oxidative stress and increased contact with sulfides.[16] Chondrichthyans, such as sharks and skates, are not able to handle the pressure of deep water as well as teleost fish, likely due to a decreased ability to regulate osmolytes..[18] Osmolytes may be replaced with piezolytes as depth increases to prevent damage from very high pressure.[18]
One example of a common and effective piezolyte is trimethylamine N-Oxide (TMAO).[16] This molecule is unique as it can work under very high pressure, while other osmolytes like glycine cannot.[19] TMAO concentrations in the muscles of deep sea fishes, crabs, and shrimp have been shown to increase as the depth of the ocean increases.[16] This was the most abundant piezolyte observed in these species.[16] TMAO has been shown to be more effective than osmolytes such as betaine, sarcosine, and proline in preventing organisms from harm in high pressure areas.[18] In sharks and skates, TMAO also helps to manage the necessary high urea levels needed to osmoregulate.[18]
The mechanism that TMAO uses to bind to water molecules stops the water from entering the interior of proteins and prevents protein backbones from dissipating.[16] TMAO allows fish to live up to 8.4 km under the surface of the ocean, the deepest region ever inhabited by a fish.[16] There are some risks associated with the use of TMAO.[16] An overabundance of TMAO can be toxic to the organism and over-stabilize proteins, causing them to maintain an undesirable form.[18] High levels of TMAO may also damage the enzyme bromelain.[19]
There are other methods used by marine organisms to circumvent high pressure.[17] Proteins in the cytosol that are responsible for binding hydrophilic ligands may undergo residue changes to adapt to high pressure environments.[17] These residue shifts help to make the proteins hydrophobic, preventing them from losing their shape.[17] One enzyme that undergoes amino acid shifts to account for higher pressure is Lactate Dehydrogenase (LDH).[17]
References edit
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